1. Field of the Invention
The present invention relates to a signal processing circuit for a cathode ray tube (CRT) display to process so that the black level of a displayed picture becomes constant. (International Patent Classification H04N 9/73).
2. Description of the Prior Art
In a display using a CRT, a circuit which is usually used to keep the black level of a displayed picture constant is called an automatic cut-off circuit. This circuit adds a detecting pulse on a vertical blanking period of a video signal. A cathode current during the vertical blanking period is detected and is processed to be kept at a constant level.
Because a pulse is added on the vertical blanking period and a detecting current flows in the CRT's cathode, when the vertical sync signal is temporarily lost, for example during video signal switching, the picture appears to be disturbed in that the pulse added period is displayed for a moment. This is a problem. In the case of a front projection display, the detecting pulse cannot be processed to prevent display on the screen and the pulse is typically displayed at the top or bottom part of the displayed picture. This is also a problem. To address the above problems, Japanese Patent Laid-Open 6-95766 discloses a circuit to process the detecting pulse not to be displayed on the screen by disabling the automatic black level stabilizing process.
A block diagram of a principal part of a circuit described in Japanese Patent Laid-Open 6-95766 is shown in FIG. 7. The circuit includes a CRT 1, a video amplifier 2, a detecting pulse adding circuit 3, a current detecting circuit 30, a DC level control circuit 54 and a control CPU 58 to control each of the above circuits. The automatic black level stabilizing function is realized by negative feedback processing that adds a detecting pulse to a video signal at detecting pulse adding circuit 3, a current detected at the cathode is converted into a voltage signal, which is A/D converted. The converted signal is sent to an interface circuit 57 and DC level control circuit 54 is controlled by control CPU 58 so that the detecting current is constant. When a video signal is switched, for example by a channel selection, display of the detecting pulse is prevented by stopping the output of the added pulse by control CPU 58. Because the same control signal as that of the negative feedback processing case is sent to DC level control circuit 54 at this time, the black level of the displayed picture is maintained.
In Japanese Patent Laid-Open 6-95766, negative feedback is processed by software and another circuit, shown in FIG. 8, is used for high speed processing. The circuit includes a CRT 1, a video amplifier 2, a detecting pulse adding circuit 3, a current detecting circuit 30, a comparison circuit 7, a switcher 10, and a data control CPU 31. In the system shown in FIG. 8, negative feedback processing is done by hardware in real time. The detecting current level is kept constant, the black level is stabilized and is detected at a sample hold circuit 5. The detected signal is compared with a reference voltage at comparison circuit 7 and is used for a grid voltage of CRT 1. Thus, the detecting current is controlled so as to be kept constant. The output of comparison circuit 7 is supplied to data control CPU 31 after being converted from an analog signal into a digital signal at an A/D converter 15 and, a voltage signal equal to the output of comparison circuit 7 is output from a D/A converter 17 and supplied to a switcher 10. To stop the automatic cut-off function, switcher 10 is switched, the grid voltage is controlled by the output of D/A converter 17 and the black level of the displayed picture is kept constant.
The circuits described above have the following problems.
(1) When the automatic black level stabilizing function is disabled (off), the black level varies over time due to factors such as environmental temperature variation and heat dissipation of the circuit elements because negative feedback processing is not done. PA1 (2) When the automatic black level stabilizing function is switched from an on-state (working) to an off-state (non-working), an error occurs in the black level due to errors at the A/D converter and the D/A converter (for example, errors in the power source voltage or unevenness of circuit constants). PA1 (3) When the automatic black level stabilizing function is switched from an off-state to an on-state, an undesirable picture results due to the appearance of an unstable black level until the negative feedback loop converges.